Shock tubes, which are also referred as non-electric detonators, are a type of low energy signal fuses. They have been widely used for connecting and initiating explosive charges in the mining and quarrying industries. They represented a revolution in the market of detonation accessories, due to the ease of connection and application, and to the intrinsic safety against accidental ignition by induction of spurious electric current. There are various types of shock tubes use high explosives as components.
Plastic extrusion may be adopted to form circular tube with an outer diameter varying from 2.0 to 6.0 mm and an inner diameter varying from 1.0 to 5.0 mm. A secondary explosive powder, such as HMX, RDX or 5 PETN, has been introduced into its inner periphery during formation of the tube. The resulting product is known as a non-electric shock tube. When initiated by a primary explosive blasting cap, a conventional shock tube generates a gaseous shock wave with a signal transmission speed ranging from 1,800 to 2,200 m/s. Further improvements may include the addition of aluminum to increase specific energy and utilization of ionomeric polymers, like Surlyn®, to increase adhesiveness of the powder. Conventional shock tube has two layers: an inner layer made of a polymer which provides adhesiveness to the explosive powder mixture, and an outer layer made of a polymer which provides mechanical strength. Surlyn® is most suitable for the inner polymer layer; and polypropylene, polyamide, or polybutene is used for the outer layer. This product was an improvement as Surlyn® alone is expensive and has a low resistance to external damage.
There are also single layer shock tubes, in which the polymer is Linear Low Density Polyethylene (LLDPE) with minor quantities of an adhesive promoter. The tube is made by extrusion of an initial tube with outer and inner diameters greater than that of the final tube. Then the tube is stretched in order to orient the LLDPE molecules, making a final tube with greater tensile strength. However, the best conventional shock tubes continue to be made in two layers, and the inner layer continues to be Surlyn®, as even a low dislodgement of poorly adhered explosive powder may lead to failures in signal propagation due to discontinuities in the powder layer or by concentration of loose powder in the lower parts of the tube during field application. Alternatively, an additional thin layer of a hydrophilic polymer, like such as PVA, is deposited by passing the plastic tube through a solution of polymer in a liquid, e.g. water, and drying the solvent. The aim is to make the tube less permeable to the hydrocarbons present in an emulsion explosive. Hot diesel fuel is particularly aggressive to LLDPE, and prolonged contact of the tube with hot, diesel fuel-based emulsions causes failure in signal propagation. The PVA protective skin is fragile and does not adhere well to the LLDPE, and so a pretreatment with a cleaner, such as chromic acid, with hot air or with an adhesion promoter is necessary.
However, the foregoing products have a number of deficiencies. The production of the tube loaded with explosives (RDX, HMX or PETN are toxic and dangerous) is associated with risks both of accidental explosion and in the handling toxic products, requiring special care and protection in the production line. The fact that molecular explosives are used impedes the dosing of non-active components during the extrusion of the tube. In addition, in the conventional shock tube, the reaction products are basically hot gases which, when leaving the final extremity of the tube, expand with loss of heat, such heat loss inhibiting the ignition of the pyrotechnic delay mixture. Slower delay powders are particularly insensitive to the shock tube output. It is therefore necessary either to add an additional column of a sensitive pyrotechnic mixture to give continuity to the explosive train or to use pyrotechnic mixtures more sensitive to heat and with larger column length. As a consequence, the final product has greater production costs, and the processing and handling of the pyrotechnic mixture entails significant accidental ignition risks.
Moreover, the adherence of crystalline explosives (RDX, HMX or PETN) in plastic tubes is low, demanding special manufacturing processes and the use of special, and expensive, polymers, usually ionomeric polymers such as Surlyn®, in order to minimize the concentration of loose powder in portions of the tube and to insure uniformity of distribution. Lack of adherence of LLDPE is particularly noteworthy. It is significant that the best known commercial brands of shock tube continue to use a two layer tube, with Surlyn® as the inner layer, in spite of the efforts to improve polymer adhesiveness by changes in polymer formulation.
Further, conventional shock tube loading lacks sufficient critical mass and critical diameter to properly propagate a shock wave by classical detonation theory. The shock wave is continuously sustained by dust explosion of the explosive powder dislodged by deformation of the plastic duct caused by the shock wave behind the reactive front. Due to the foregoing feature, a conventional shock tube fails if there is a cut or a close restriction in the inner duct, dispersing the shock wave. In the field practice, if unexpected cuts, stretching, knots, holes, or closed kinks unexpectedly appear in the tube, the tube can fail to propagate. Conventional shock tubes can fail to propagate after prolonged underwater exposure above 2 bar pressure, as is often found in field practice, due to the hydrophilic characteristics of the ionomeric resins like Surlyn®.
Conventional shock tubes are sensitive to the effect designated in the industry as “snap, slap, and shoot”. An unexpected ignition can occur if the tube is stretched causing rupture, in particular conditions of mechanical energy release. Conventional shock tubes are classified for transport purposes as an explosive in many countries, which results in additional costs and difficulties for transportation, especially after the increase in dangerous products regulations resulting from the fight against terrorism. Tubes manufactured with Surlyn® alone have a low tensile strength, and a low resistance to abrasion, kinks, knots, etc., demanding co-extrusion of an additional outer layer of polyethylene. This improved process still includes the use of expensive Surlyn®.
Conventional explosive powders lack sufficient activation energy to propagate in case of contamination of the tube interior by hot hydrocarbons (most likely diesel fuel) from explosive emulsions. Polymers, including LLDPE, are quite susceptible to aggression. Minor quantities of adherence-improving additives, typically EVA copolymers, are even more subject to attack by volatile fractions of diesel fuel. An additional skin of hydrophilic polymer like PVA is needed, but abrasion resistance of the skin, particularly in the rough environmental conditions found in field practice, is remarkably bad, causing removal of the skin and failures of the tube.
Conventional shock tube speeds of deflagration range from 1,800 to 2,200 m/s, or within 10% of a mean speed of 2,000 m/s. On the other hand, the electronic delay blasting cap is characterized by its highly accurate electronic delay element. As a result, when using a conventional shock tube to initiate such as electronic delay blasting cap, it was possible that the high speed of the shock tube interferes with the accuracy of the delay element. In other words, the relatively broad range interferes with the accuracy of the delay element timing. The timing error of a certain length of tube is added to the intrinsic timing error of the electronic circuit. In a typical tube length of 21 m, as used in open pit mining, the error would be within +/−1 ms, while the intrinsic error of the electronic circuits is typically within +/−0.1 ms. Conventional shock tube deflagration generates substantially gaseous reaction products, sustaining a shock wave that quickly disperses most of the released thermal energy, through the expansion of the gases as they leave the tip of the tube. For this reason, a conventional shock tube output is unable to ignite low flame sensitive delay mixtures, demanding an additional, highly flame sensitive, igniter element for ignition of the slower delay elements. Highly flame-sensitive mixtures are usually also highly sensitive to mechanical shock, friction and electrostatic discharge, increasing the risks of accidental detonation. The additional element also increases the manufacturing costs.
A further development in low energy transmission fuses was the tubes that make use of pyrotechnic mixtures inside the tube, as substitutes for the high-explosive-containing powders. A circular tube formed via plastic extrusion with an outer diameter ranging from 2.0 to 6.0 mm, and an inner diameter ranging from 1.0 to 5.0 mm. A powder of pyrotechnic mixture of K2Cr2O7+Al or Mg, Fe2O3+Al or Mg, or Sb2O3+Al or Mg, Sb2O5+Al or Mg or O2+Al or Mg, is introduced in the inner periphery of the tube during formation of the tube. The resulting product is designated as a pyrotechnic shock wave tube. When initiated by a primary explosive detonator, such a tube generates an aluminothermy reaction without gas releases, and develops plasma for energy transmission. Signal transmission tubes are usually complemented with the insertion of a delay blasting cap in the tip of the tube. The blasting cap is made of a metal cap containing two layers of explosive powder pressed inside. The bottom layer is a secondary high explosive, and the upper layer is a primary, flame sensitive explosive. The cap further includes a delay element consisting of a metallic cylinder containing in its interior a compacted column of powdery pyrotechnic delay mixture and, frequently, an additional column of pyrotechnic mixture sensitive to the heat generated by the tube's shock wave.
However, a typical pyrotechnic shock tube has its own drawbacks. The pyrotechnic mixtures use toxic components such as K2Cr2O7 and Sb2O3 and flammable solvents. It demands recycling of the solvents, and creating handling issues and requiring appropriate waste disposal. The process of extrusion of the plastic tube includes the dosing of a previously prepared sensitive pyrotechnic mixture during the formation of the plastic tube, with safety risks in handling and processing. A pyrotechnic shock tube does not resist aggression from the hydrocarbons present in emulsion explosives, and prolonged exposure leads to failures in propagation.
In addition, due to the high Tammann temperature of the components, the mixture of Fe2O3+Al or Mg were also not shown to be feasible in practice, because of low sensibility of these pyrotechnic mixture to the ignition stimulus of blasting caps and a high rate of propagation failures. Mixtures of O2+Al or Mg were not shown to be feasible in practice, due to the loss of gases in the production and use of the product. On the other hand, the formulation of K2Cr2O7 and Sb2O3, Sb2O5 with Al or Mg is highly toxic and highly friction and shock sensitive. Further, reaction products formed in the aluminothermy reactions have low thermal conductivity, which inhibits the ignition of slower, low sensitive delay elements. Moreover, the powdered pyrotechnic mixture also presents a low adherence to the tube polymer, particularly in LLDPE. Pyrotechnic mixtures are not optimized to allow propagation through closed knots, cuts or kinks.
Therefore, the present invention is aimed to solve the above described deficiencies, thus to provide a safe and effective shock tube.